The correct answer is A. DOS.
DOS stands for Disk Operating System. It is a single-tasking operating system, which means that it can only run one program at a time. This is in contrast to multitasking operating systems, which can run multiple programs at the same time.
Windows, Unix, and Linux are all multitasking operating systems. Windows is a popular operating system for personal computers. Unix is a popular operating system for servers and workstations. Linux is a free and open-source operating system that is used on a variety of devices, including personal computers, servers, and smartphones.
Single-tasking operating systems are less complex than multitasking operating systems, and they are often used on devices with limited resources, such as embedded systems. However, multitasking operating systems are more versatile and can be used on a wider range of devices.